This invention relates to controlled current ac motor drives, and more particularly to a feedback control and method of substantially reducing the cogging torque produced by controlled current drive systems at low frequencies.
Many applications including traction drive systems require the precise regulation of motor torque. The development of current source or controlled current inverters, which supply rectangular non-sinusoidal currents to the motor windings, has resulted in efforts to apply this device to adjustable speed ac induction motor drives. One of the weaknesses of present control strategies is that the torque pulsations due to the harmonic or cogging component of electromagnetic torque can be severe at very low machine frequencies and result in instabilities and uneven running. For a six pulse, polyphase full wave bridge inverter, torque ripple occurs because of the presence of the sixth, twelfth, and eighteenth harmonic components in the non-sinusoidal motor current in addition to the fundamental motor frequency, which is the electrical equivalent of the mechanical speed (RPM) at which the shaft is rotating. The torque pulsations are especially troublesome upon starting up or when passing through zero speed to reverse the direction of rotation, and can be eliminated by modulating the dc link current fed to the inverter.
In practice, motor parameters vary with temperature and frequency so that actual real-time measurement of the pulsating torque and closed-loop feedback control is necessary for the precise regulation of torque rather than relying on open loop compensation. An open loop technique for small industrial drives is described in U.S. Pat No. 4,066,938 to F. G. Turnbull, entitled "Input Current Modulation to Reduce Torque Pulsations in Controlled Current Inverter Drives," and assigned to the same assignee as this invention. A closed loop technique for reducing torque ripple requiring the continuous calculation of actual torque from the sensed motor voltage and current is disclosed in U.S. Pat. No. 3,919,609 to Klautschek et al.; in this patent the actual torque developed by the machine is compared to a predetermined reference value and the error signal is used to modulate the dc link current in a corrective sense. One disadvantage with this approach is that in practice it may be required to regulate a motor parameter other than machine current by varying the dc link current magnitude; another is that it is preferable to be able to switch out the cogging torque reduction control at higher machine frequencies so that the machine can properly respond to torque pulsations caused, for instance, by a sudden change in load.